Xenix

Xenix is a discontinued Unix operating system for various microcomputer platforms, licensed by Microsoft from AT&T Corporation. The first version was released in 1980, and Xenix was the most common Unix variant during the mid- to late-1980s. The Santa Cruz Operation (SCO) acquired exclusive rights to the software, and eventually replaced it with SCO UNIX, later known as OpenServer, with the final Xenix version released in 1991. Provided by Wikipedia